The present invention relates to a control apparatus for an automatic transmission that can improve a downshift control to be executed in response to a driver's intent to decelerate, and also relates to a related control method.
The automatic transmissions for automotive vehicles include a hydraulic pressure control circuit to select engagement/disengagement of a plurality of frictional engaging elements, such as hydraulically controlled clutches and brakes, to realize a plurality of gear stages. According to these automatic transmissions, when a vehicle is traveling on a descending road, a sufficient engine brake force may not be obtained even when an accelerator is operated to an OFF state. In such a case, a driver will turn off an overdrive switch or manipulate a shift lever to cause the transmission to perform a downshift operation from a D-range to an S-range and to an L-range, thereby increasing an engine brake force.
When a downshift operation for increasing an engine brake force is performed in such an accelerator OFF state in response to a driver's intent to decelerate (i.e. decelerating operation), a gear ratio of the automatic transmission increases and accordingly it is necessary to increase the rotational speed of an engine. However, in a driving mode requiring such an engine brake, as a throttle valve is closed, the frictional engaging elements activated to establish an intended gear stage in this downshift operation will transmit a torque of an output side to the engine. Thus, the engine rotational speed will increase. Accordingly, a shift time will be long. A required engine brake will not be obtained at a desired timing. An inertia torque caused due to increase in the engine rotational speed will appear as a braking torque of a vehicle. This will temporarily increase the engine brake force and cause a shift shock. Furthermore, abruptly increasing a transmission torque of a frictional engaging element to shorten a shift time, for example by controlling a hydraulic pressure of the automatic transmission, will quickly increase the engine rotational speed and accordingly the braking torque will abruptly increase and a large shift shock will occur.
To solve this problem, Japanese Patent No. 2924463 (referred to as Prior Art Document 1) discloses a control technique according to which an engine output increasing means for temporarily increasing an engine output when an automatic transmission is shifted down to a low-speed gear stage that is capable of producing an engine brake in a condition that an accelerator is in an OFF state. During the downshift operation, a timer measures a time elapsed from predetermined measurement start timing, such as shift output timing for switching a hydraulic pressure control circuit. During a downshift operation, a slip occurs in a frictional engaging element of a high-speed gear stage to be disengaged. Meanwhile, a frictional engaging element of a low-speed gear stage starts engaging during this downshift operation. The engine rotational speed must be increased until the frictional engaging element of a low-speed gear stage is completely engaged. The engine output increasing means starts an engine output increasing control based on the elapsed time measured by the timer. The start timing of this engine output increasing control is set with reference to vehicle operating conditions (e.g. oil temperature in a hydraulic pressure control circuit, or engine rotational speed) that give effects on an engagement/disengagement delay time of the frictional engaging element or a delay time in increasing the engine output.
Furthermore, there is a delay time before the frictional engaging element of an automatic transmission is actually disengaged or engaged. Furthermore, there is a delay time before the engine output actually starts increasing when a throttle opening control is performed to increase the engine output. The above-described Prior Art Document 1 proposes to shorten the shift time without causing any shift shock by taking these delay times into consideration in setting the start timing, and also discloses that a throttle valve opening control should be performed to cause the engine to increase its rotational speed when any slip occurs in the frictional engaging element of a high-speed gear stage.
However, the delay time required for the frictional engaging element to actually disengage or engage during a downshift operation is dependent on not only the oil temperature in a hydraulic pressure control circuit or the engine rotational speed but also a vehicle speed or a torque acting on this frictional engaging element when the downshift control is executed. Especially, an accelerator pedal is substantially fully closed during a downshift operation. It is necessary to consider that an arbitrary driving torque of a road load (i.e. a torque required to maintain a cruise running condition at the present traveling speed) or less is added from the engine to the transmission. Furthermore, the delay time will be dependent on the operating conditions including a slip control of a lockup clutch. Therefore, relying only a timer to set the control start time is not always accurate even if the effects of the oil temperature or the engine rotational speed are taken into consideration and, accordingly, a throttle valve opening control (i.e. engine output increasing control) may not be started at appropriate timing. Hence, the throttle valve opening control possibly starts at inappropriate timing and a driver will feel badly or receive a shock caused by the throttle valve opening control during a downshift operation. Furthermore, appropriately setting reference values of a timer considering the effects of the oil temperature or the engine rotational speed requires repetitive experiments for determining these values. Moreover, such setting of reference values of a timer will be necessary again if the contents of the hydraulic pressure control for the shift operation is changed or renewed later. Not only the control logic becomes complicated but also the many parameter settings will be required.